Problem: The scale on a map is 6cm : 2km. If two cities are 30cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 6cm on the map corresponds to an actual distance of 2km. A distance of 30cm on the map is the same as 5 $\cdot$ 6cm. The actual distance between the two cities is 5 $\cdot$ 2km, or 10km.